1. Field of the Invention
The present invention relates to a method and a machine for electrical discharge machining a workpiece and providing finished surfaces of good quality. The invention particularly concerns the use of a variable capacitance and a variable inductance in connection with an impedance matching circuit of the electrical discharge machine.
2. Description of the Background Art
Generally in machining at alternating-current high frequency, it is well known that at an average machining voltage of zero (V), chipping does not occur due to electrolysis and polarity changes from one to the other every half-wave discharge. Accordingly, an excellent machining characteristic is provided which ensures that a high quality machined surface can be obtained every discharge.
For example, Japanese Laid-Open Patent Publication No. SHO61-260915 discloses an electrical discharge machining power supply which can supply a machining gap with the alternating-current high frequency of 1.0 to 5.0 MHz. The power supply can reduce the stray capacitance resulting from the sum of the capacitance existing in feeders and the capacitance formed in the machining gap (between an electrode and a workpiece) to a value equal to or less than 1000 pF. As a result, an excellent surface of not more than 1 .mu.mRmax can be obtained.
However, when the machining gap, machined area etc., vary and/or a discharge status changes in such electrical discharge machining power supply, the impedance of the machining gap changes sharply to substantially vary the output thereof. This creates a problem in that some machining might result in instability, unreproducibility, etc. In the meantime, as means to solve such a problem, Japanese Laid-Open Patent Publication No. HEI1-240223 discloses an example wherein an automatic impedance matching circuit is provided between an alternating-current power supply and the machining gap. This arrangement permits a workpiece to be machined with an automatic adjustment of impedance in response to the changes in machining gap distance and machined area.
FIG. 33 shows the arrangement of a conventional circuit, wherein the numeral 1 indicates a direct-current power supply, 2 denotes a resistor provided for limiting a current, 3 designates a stray capacitance present in feeder cables and the circuit, 4 represents a stray inductance present in the feeder cables and other mechanical structure (such as a feeding section), 5 indicates a machining gap capacitance formed between an electrode and a workpiece, 6 denotes a machining gap formed by the electrode and the workpiece, 7 designates a switching device, 8 represents a drive circuit which drives the switching device 7, 9 denotes a coupling capacitor provided in series between the switching device 7 in the circuit and the machining gap 6, 10 indicates a coupling coil provided similarly in series between the switching device 7 and the machining gap 6, 11 represents an alternating-current, high-frequency machining power supply comprising several of the aforementioned components, and 12 denotes an impedance matching circuit.
FIG. 34 shows the internal circuit arrangement of the conventional automatic impedance matching circuit 12, wherein 13 designates a coupling capacitor, 14 indicates a coil, 15 represents a variable capacitor having a selectable capacitance, 16 denotes an actuator, e.g., a motor, for changing the capacitance of the variable capacitor 15, and 17 designates a drive control circuit which drive-controls the actuator 16.
In operation, the switching device 7 is driven to perform on-off operation, thereby generating an alternating-current, high-frequency voltage as the output of the alternating-current, high-frequency machining power supply 11. The output voltage is supplied as a machining voltage to the machining gap 6 through the automatic impedance matching circuit 12 via the feeder cables to machine a workpiece. Generally, a traveling wave and a reflected wave (an oppositely directed wave reflected at an output end) exist when there is a transmission at high frequency. However, when the matching has been made completely, only the traveling wave is present to provide the maximum output. Namely, the ratio of the reflected wave to the traveling wave must be minimized in order to provide a maximum output.
A high-frequency signal entered into the automatic impedance matching circuit 12 is impedance-matched by the T-shaped matching circuit consisting of the coupling capacitor 13, the coil 14 and the variable capacitor 15 and is supplied to the machining gap 6, at which time the control circuit 17 causes the capacity of the variable capacitor 15 to be changed by the actuator 16 according to a machining status.
According to the art shown in FIGS. 33 and 34, if the impedance of the machining gap changes due to the changes in the size of the machining gap, machined area, machining status, etc., the matching is adjusted to achieve stable, excellent surface machining.
It should be noted that the workpiece must be isolated for machining as shown in FIG. 35 to provide a machined surface of approximately 1 .mu.mRmax in the conventional art. In this drawing, 11 indicates an alternating-current, high-frequency machining power supply, 12 designates an impedance matching circuit, and 18 denotes feeder cables for alternating-current high frequency, which are low-capacitance cables having the capacitance of approximately 100 pF per meter. 19 represents feeder cables for high-speed machining, which have been reduced in inductance to supply a high-peak current waveform but whose capacitance is much larger than that of the feeder cables 18. 20 indicates a high-speed machining power supply, 30 denotes a workpiece, 31 designates a wire electrode, 32 represents a clamp jig, 33 indicates a surface plate, 34 denotes feeders, 23 represents an insulating jig for isolating the workpiece 30 from the surface plate 33, and 24 designates a switch which disconnects and connects the workpiece 30 on the insulating jig 23 from and to the surface plate 33.
The switch 24 is closed in roughing to connect the workpiece 30 to the surface plate 33 whereby the high-peak current is supplied from the high-speed machining power supply 20 to machine the workpiece 30. The feeder cables 19 which supply the high-peak current are low in inductance but are generally large in capacitance. In an commonly used frequency band of approximately 2 MHz, the current flows into the capacitance of the feeder cables 19, thereby leading to difficulty in impedance matching. Also, electrostatic energy accumulated in the feeder cables 19 at the time of a discharge is discharged to the machining gap and results in an increase in the energy of a discharge current waveform, thereby deteriorating the roughness of the machined surface. For this reason, when the workpiece 30 is finished at the alternating-current high frequency, the switch 24 is opened to cause the workpiece 30 to be isolated from the surface plate 33 by the insulating jig 23. In this state, the high-peak current supplying feeder cables 19 is separated from the circuit. This will facilitate the impedance matching at the machining gap. Further, the electrostatic energy accumulated in the low-capacitance feeder cables 18 is small enough to provide the waveform of small current energy. As a result, the finished surface has good quality.
In order to finish a workpiece to a surface of good quality at alternating-current high frequency in the conventional electrical discharge machine arranged as described above, the insulating jig 23 or the like was used to isolate the workpiece 30 from the machine surface plate 33 and the switch 24 was required to disconnect and connect the workpiece 30 on the insulating jig 23 from and to the surface plate 33, which posed problems of machining accuracy, operability and costs.
Also, when the insulating jig 23 is used for alternating-current, high-frequency machining of a workpiece immersed in dielectric fluid, a capacitance is formed between the workpiece 30 and the surface plate 33 via the dielectric fluid and acts to deteriorate the machining quality.
Further, in die-sinking electrical discharge machining which uses an electrode having a large area, if the insulating jig 23 is employed to isolate the workpiece 30 from the machine surface plate 33, the large capacitance formed between the electrode and the workpiece causes machined surface roughness to deteriorate, whereby a machined surface of good quality cannot be provided.
Also, since the variable capacitor 15 was varied by the actuator 16 in order to provide matching in the impedance matching circuit 12 of the conventional electrical discharge machine, the machine was complicated, the fitting of the circuit was difficult, and the costs were high.
Also, particularly when a machined area varies greatly or power supply frequency changes in the electrical discharge machine, it is necessary to switch between a plurality of inductances in the impedance matching circuit 12. Since this switching was also designed to be complementary to that of said variable capacitor 15, the machine was complicated, the fitting of the circuit was difficult, and the costs were high.
It is accordingly an object of the present invention to overcome the disadvantages in the conventional art by providing a method and a machine for electrical discharge machining which can eliminate the influence of capacitances formed in high-speed machining feeder cables and formed between an electrode and a workplace upon a machining gap, provide machined surfaces of good quality, and improve operability and cost performance greatly.
It is another object of the present invention to provide a low-cost, compact, variable-capacitance apparatus of high accuracy which can form a low-level capacitance easily and has high accuracy.
It is a further object of the present invention to provide a low-cost, compact, variable-inductance apparatus of high accuracy which can design and form a low-level inductance easily and has high accuracy.